The invention pertains to a device for the modulation of the illuminating light of an electro-optic display screen, and especially to a modulating device for a display screen working n transmission.
One of the problems encountered in flat light modulating screens, such as liquid crystal screens is the 10, difficulty of modifying the mean brilliance of these screens to observe them in highly variable light environments (from 100,000 lux to a few lux, for example).
The most obvious solution, in terms of its principle, is the one in which the light intensity of the lighting source, associated with this screen, is made to vary by a modification of the voltage or supply frequency of this source.
For flat screens, three types of lighting are generally used:
(a) Incandescent lighting,
(b) Electro-luminescent lighting,
(c) Fluorescent lighting.
Let us examine the possibility of modifying the luminous flux of these lightings:
(a) For incandescent lighting (whether halogenic or not) it is observed that:
The luminosity power can be made to vary continuously by modifying the supply voltage.
By contrast, this variation, which is accompanied by a modification in the color temperature, is unsuited to color screens.
(b) For lighting by electro-luminescence, this effect can easily be used to make a source of plane light, the luminous flux of which can be made to vary (by about one decade) by modifying the supply voltage. This type of source has two disadvantages:
The light intensity/voltage curve exhibits a threshold and is non-linear.
The light is emitted in a narrow spectral band.
(c) Finally, for a fluorescent lighting system:
The effect used, which is a discharge in a gas, has a voltage threshold below which it is impossible to light up the fluorescent tube. It is therefore very difficult to vary the luminous flux by varying the supply voltage. By contrast, a variation of the excitation frequency can be used to vary the fluxes emitted to a small degree.
The invention pertains to a modulation device which uses any type of light and avoids the disadvantages referred to above.